This invention pertains to an improvement in protectively coated flanged pipe. This invention pertains also to a method for making such pipe.
Metal pipe intended for corrosive or erosive applications is often provided with a plastic or resin coating for protection of the base metal. Typically, the base metal is stainless steel or a conventional carbon steel, and the coating is a resin, such as polyvinyl chloride (hereafter PVC). While a variety of methods may be used for coating the pipe, dip coating is preferred for simplicity and to ensure complete coating of all of the exposed surfaces of the pipe, both inside and out.
When the typical resin coating, deposited in this manner, is squeezed between the bolt heads, washers, and nuts of a flanged pipe joint, the resin, particularly dip coated PVC which is relatively soft, tends to "creep" or "cold flow" over a period of time. Thus, the integrity of the seal deteriorates as a result of loosening of the fastening hardward, unless the fastening hardware is tightened periodically. In many applications, this is intolerable.
Heretofore, this problem has been rectified by the provision of thin stainless steel plates disposed on the facing outer surfaces and abutting flange joints. Thus the pressure on the flanges is evenly distributed on the underlying PVC coating, and the tendency of the coating to creep is minimized. This solution to the problem, however, is quite expensive.
Ideally, of course, the plastic coating on the flange surfaces should be sufficiently hard and free of the tendency to creep, while protecting the underlying pipe. To provide such a coating on the entire pipe surface, including that part of the pipe surface other than a flange, however, has been considered impractical or overly expensive.
For purposes of considering the patentability of the invention disclosed and claimed herein, a brief patentability search was conducted. The U.S. Pat. Nos. identified to be of possible interest in that search were: 3,248,253 Barford et al, 3,392,700 H. E. Mallory, 3,425,455 Kilpert et al, 3,502,492 L. L. Spiller, 3,541,670 E. D. McCrory, 3,687,704 R. C. Stanley, 3,946,125 W. Scheiber, 3,965,854 W. Scheiber, 4,075,376 F. Jaeger, 4,122,798 J. E. Gibson, 4,273,798 W. Scheiber
It should be noted that the Scheiber '125 patent is directed to an apparatus and method for coating hollow bodies, and it does show a coating applied to pipe bodies in stages where the first stage coats the central body and the second stage coats the end sections.
The Stanley patent also shows a method for coating pipes using a two stage type coating, in the first stage of which an initial layer of coating material is applied longitudinally to the pipe. At the second coating station, a second layer of coating is applied.
The Scheiber '798 patent also shows coating of a pipe in a plurality of stages but does not show the seperate coating of the pipe flange.
A pipe coating method wherein spraying and immersion coating is combined is taught in the Mallory patent. However, this is not accomplished sequentially or with reference to selectively coated pipe surfaces, as in the present invention.
In general then these references are not deemed to suggest or render obvious, much less anticipate, the present invention.
Having in mind all of this prior art and the problems referred to above, the general object of the present invention is to provide a protectively coated flanged pipe coated substantially in a relatively conventional and practical way but including a coating of improved characteristics with respect to the maintenance of joint integrity. It is also a part of the general object of the present invention to provide a method for making such an improved pipe.